1. Field of the Invention
The invention relates to a method for forming a surface mounted device (SMD)-type piezoelectric vibrator in which a piezoelectric vibrating reed is sealed in a cavity formed between two bonded substrates, a piezoelectric vibrator manufactured by the manufacturing method, and an oscillator, an electronic device, and a radio-controlled timepiece each having the piezoelectric vibrator.
2. Description of the Related Art
Recently, a piezoelectric vibrator utilizing quartz or the like has been used in cellular phones and portable information terminals as the time source, the timing source of a control signal, a reference signal source, and the like. Although there are various piezoelectric vibrators of this type, a surface mounted device-type piezoelectric vibrator is known as one example thereof. As the main piezoelectric vibrator of this type, generally, a three-layered piezoelectric vibrator in which a piezoelectric substrate having a piezoelectric vibrating reed formed thereon is interposed between the base substrate and a lid substrate from above or below is known. In this case, the piezoelectric vibrating reed is accommodated in a cavity (sealed space) that is formed between the base substrate and the lid substrate.
Moreover, in recent years, instead of the three-layered piezoelectric vibrator, a two-layered piezoelectric vibrator has also been developed. The piezoelectric vibrator of this type has a two-layered structure in which a base substrate and a lid substrate are directly bonded, and a piezoelectric vibrating reed is accommodated in a cavity formed between the two substrates. The two-layered piezoelectric vibrator is ideally used, as it is superior for achieving a thin profile compared with the three-layered structure.
As a piezoelectric vibrator of this type, as disclosed in Patent Document 1 as below, a configuration is known in which penetration holes are provided in a substrate (base substrate) formed of glass or ceramics, a wiring metal is formed on the inner surfaces of the penetration holes and the upper and lower surfaces around the penetration holes or either of the surfaces, alloys are welded to the penetration holes to form airtight terminals, and a crystal reed (piezoelectric vibrating reed) provided on the substrate surface is electrically connected directly to the alloys of the airtight terminals or through the wiring metal on the substrate surface. According to this configuration, a predetermined driving voltage can be applied to the piezoelectric vibrating reed from the outside of the piezoelectric vibrator through the airtight terminals, namely penetration electrodes which are electrodes formed so as to penetrate through the substrate.
Moreover, in the piezoelectric vibrator, the substrate and a cover (lid substrate) provided to the substrate are anodically bonded together by an anodic bonding metal film formed on the cover. As an example of an embodiment in which the substrate and the cover are anodically bonded together in such a way, a method is known in which a substrate and a cover superimposed onto each other with a metal film disposed therebetween are set on an electrode substrate of an anodic bonding machine, and a predetermined bonding voltage is applied between the metal film and the electrode substrate. According to this method, it is possible to cause an electrochemical reaction to occur at the interface between the metal film and the substrate. As a result, the metal film and the substrate are closely adhered tightly, whereby the substrate and the cover are anodically bonded together.
Meanwhile, in piezoelectric vibrators, it is generally preferable to suppress an equivalent resistance value (effective resistance value Re) to a low value. Since a piezoelectric vibrator having a low equivalent resistance value is capable of vibrating a piezoelectric vibrating reed with a low power, a high-quality piezoelectric vibrator having high energy efficiency can be achieved. Moreover, as the inside the cavity in which the piezoelectric reed is sealed approaches/is close to a vacuum, that is, as the degree of vacuum in the cavity increases, the equivalent resistance value can be suppressed to be lower is generally known.    Patent Document 1: JP-A-6-283951
However, the piezoelectric vibrator of the related art and the manufacturing method thereof have the following problems.
First, although the bonding film used for anodic bonding is generally formed by photolithography, since the apparatus needed for the photolithography is expensive, there is a problem in that bonding the base substrate and the lid substrate by anodic bonding incurs large costs.
Another problem is that when the base substrate and the lid substrate are bonded by anodic bonding, since a reaction gas mainly composed of oxygen is generated when an electrochemical reaction occurs at the interface between the bonding film and the substrate during the anodic bonding. Therefore, the reaction gas is sealed in the cavity by the bonding, and the degree of vacuum of the cavity decreases, which influences the quality of the piezoelectric vibrator.
The invention has been made in view of the above problems, and an object of the invention is to provide a high-quality, two-layered surface mounted device-type piezoelectric vibrator which can be manufactured at a low cost and in which a decrease in the degree of vacuum in the cavity is suppressed and a method for manufacturing the piezoelectric vibrator. Moreover, another object of the invention is to provide an oscillator, an electronic device, and a radio-controlled timepiece each having the piezoelectric vibrator.